1. Field of the Invention
The present invention relates to a thin film transistor and a method for manufacturing the thin film transistor, and a semiconductor device and a display device using the thin film transistor.
2. Description of the Related Art
As a kind of field-effect transistor, a thin film transistor in which a channel formation region is formed in a semiconductor layer formed over a substrate having an insulating surface is known. Techniques in which amorphous silicon, microcrystalline silicon, or polycrystalline silicon is used for the semiconductor layer used in the thin film transistor have been disclosed.
Further, a technique for increasing the amount of current flowing between a source and a drain by forming contact layers in contact with a source electrode layer and a drain electrode layer (also referred to as a source region and a drain region) of a thin film transistor with the use of an n-type microcrystalline silicon has been disclosed (see Patent Document 1).
On the other hand, the size of a transistor has been reduced in order to improve resolution of a display device and increase the aperture ratio in a pixel, and a thin film transistor with a short channel length which is approximately an exposure limit of a light-exposure apparatus (MPA) has been considered. In a thin film transistor with a short channel length, an on current can be increased and a threshold voltage can be reduced.    [Patent Document 1] Japanese Published Patent Application No. H3-185840